Frameshift Mutations

While point mutations involve the changing of one letter in the DNA sequence into another letter, they do not involve the loss or gain of letters in the sequence. A codon may change but all other codons remain intact.

A frameshift mutation is much more serious. Since codons are read in groups of 3, any LOSS of a single nucleotide would change the way all downstream letters are grouped into codons. This would shift the reading frame of all the codons and the sequence of amino acids in the chain would be dramatically altered. The same would happen if a letter were inserted into the sequence.

There are a number of causes of frameshift mutations. A common one happens when cells are exposed to UV rays from the sun. When a cell is hit with UV rays any sections of DNA that have a pair of T’s side by side will cause these nucleotides to bond to each other rather than to the pair of A’s on the opposite strand. This Thymine Dimer formation causes bumps in the DNA. Normally enzymes would break up these thymine dimers and restore the shape of the DNA strand.

If, however, too many of these form and the enzymes become overworked, they remain in the sequence. When DNA polymerase comes along for a round of replication, the T T combination makes it difficult to place an A A combination in the new strand. If the polymerase is confused it may only place on nucleotide in the strand instead of two. This results in the loss of one letter in the sequence and gives a frameshift mutation.

Aflatoxin is a known carcinogen that is commonly found in peanut butter and it also causes frameshift mutations. (of course you need to consume pounds of it a day before it becomes problematic and if you're eating that much peanut butter you've got bigger problems) The dose found in peanut butter is remarkably low but it is toxic to the liver. A number of other dangerous chemicals can cause mutation. For this reason the FDA requires testing before any new chemical is used in food.